1. Field of the Invention
The present invention pertains to the field of wavefront sensing and, particularly, to an improved ophthalmic aberrometer and method for retinal illumination.
2. Description of Related Art
A wavefront sensor, often referred to as an aberrometer (which terms will be used interchangeably herein), is a device that measures a difference in the optical path of light between a deformed wavefront and an ideal or reference wavefront. A properly processed measurement yields values for various aberrations in the optical system that the light propagates through. Recent attention has focused on the design and use of wavefront sensors for measuring the aberrations of the eye with the goal of improving visual quality. Williams"" U.S. Pat. No. 5,777,719 describes a Shack-Hartmann type wavefront sensor that can be used to measure, among other parameters, higher-order ocular aberrations. Shack-Hartmann wavefront sensors are characterized by a microlens (lenslet) array for imagining the light reflection from the retina into an array of spots on a detector. The resulting spot image array is displaced from the regular array resulting from an unaberrated optical system. These displacements of the spots are used to determine the wavefront slope at each spot location and this information is typically used to determine the coefficients of Zernike polynomials which represent different orders and modes of the wavefront aberrations. Other types of aberrometers include the spatially resolved refractometer based on the Scheiner optometer, those based on the Tscheming principle, skiascopic systems, scanning systems of the Tracey technology type, raytracing devices, and others. All of these aberrometer types are well known in the ophthalmic wavefront sensing art so that a detailed description of these devices is not necessary to understand the invention. Descriptions of these devices can be found, for example, in J. Refractive Surg. 16 (5), September/October 2000.
Regardless of the sensing principles of different type aberrometers, they all require a retinal illumination source. This is typically a light emitting diode, a superluminescent diode (SLD), a diode laser (typically operated below threshold) or, another, preferably partially-coherent source that produces a point source on the patient""s retina. In fact, it is highly desirable that the eye illumination focus on the eye""s fovea so that the ultimate wavefront measurement represents aberrations at the fovea, the highest resolution portion of the retina. Illumination that covers an area larger than the fovea will produce less accurate aberration values. Generally, patient refractive error is the largest optical defect to contend with in aberrometer wavefront measurement. Such error limits the measurement range of the aberrometer. The typical ophthalmic patient will have an uncorrected defocus in a wide range between xc2x115 diopters (D). This means that the eye will focus light some distance either in front of or behind the retinal plane, producing blurry images on the retina when this value is different from zero.
Lasers (used herein throughout to refer to the retinal illumination source) used for providing the retinal illumination typically have beam diameters of about 1.5 mm. Since the fovea of the eye is also about 1.5 mm in diameter, any defocus power in the eye will inhibit a tight match between the illuminating beam and the retinal target. Aberrometers are generally constructed such that their optical systems include refocusing means to account for the patient""s refractive power, and also, so that the wavefront image spots are in focus on the wavefront detector. The refocus of the laser beam can be accomplished by injecting it in a position in the aberrometer optical system so that the refocus occurs with the correction of the patient""s defocus. Alternatively, a separate focussing optical path can be provided for the illumination light. These solutions require that the laser beam pass through refracting optics (lenses). The principal drawback, however, is the noise generated in the wavefront sensor from backscatter due to the inherent disparity in light intensity between the light entering and exiting the eye. For 780 nm light, for example, approximately 0.1% of the illumination light is collected for wavefront imaging. The solution provided by polarization optics is too costly to be effective.
Another concern for accurate wavefront measurement is the compensation of refractive errors on the input side of aberration measurement. One approach for providing a small illumination spot on the fovea was to create a best focus by geometrically correcting the input light by either adding or subtracting optical power from a plane wave. Hence, the input light would diverge or converge to compensate for myopia or hyperopia, respectively. In a myopic eye with a small pupil diameter, however, the input beam is diverging before intercepting the cornea, and the input light profile can take on a significant aberration signature prior to striking the retina. This can seriously degrade the intensity profile distribution which can interject error in localizing (centroiding) the imaged spot on the fovea and, in turn, in the wavefront reconstruction. Moreover, a very small input beam will suffer from diffraction effects and reduce measurement range.
Spot asymmetry on the fovea is a further concern affecting accurate wavefront measurement. Poor or inaccurate localization of imaged lenslet spots can create errors in the Zernike polynomial terms of the reconstructed wavefront. Since a Shack-Hartmann device senses a differentiated wavefront, the Zernike terms are no longer mutually independent (i.e., non-orthogonal). As such, system noise can induce Zernike term cross-coupling leading to artificially created Zernike quantities that do not actually exist.
Accordingly, the inventor has recognized a need for a retinal illumination apparatus in an aberrometer and associated method that address the disadvantages of the current technology. These and other advantages and objects of the present invention will become more apparent in view of the following description and figures.
The invention is generally directed to illuminating a patient""s retina for making a wavefront measurement with an illumination beam having a beam characteristic, e.g., diameter or profile, that a) eliminates the need to refocus between the source and the patient""s cornea, and b) which maintains a beam spot area on the fovea that is smaller than the diffraction limit of a wavefront imaging component over a defocus range typically encountered in the patient population; i.e., between about xe2x88x9212D to +10D and, preferably, between xe2x88x9212D (xc2x10.25D) to +6D (xc2x10.25D). An embodiment of the invention is directed to an improved wavefront sensing device. The improvement is characterized by the aberrometer having an optical path between a retinal illumination source and a patient""s eye containing no refractive, diffractive, or other phase altering components. In other words, only beam steering components, if any, are present in the optical path between the retinal illumination source and the patient""s eye. Thus, the effective use of Gaussian wave propagation will provide a tight beam waist and a Rayleigh range that extends over a specified refractive error range. Preferably, the beam diameter of the illumination beam at the patient""s anterior cornea is less than 1 mm. The retinal illumination source is preferably a 780 nm diode laser assembly including an integrated collimating lens; alternatively, a SLD or other source producing coherent or semi-coherent light of a suitable wavelength, plus fixed lens component, may provide the appropriate illumination size and profile. The wavefront imaging component for imaging at least a portion of the unknown wavefront on a detector is preferably a lenslet of o microlens array of a Shack-Hartmann sensor.
In another embodiment, a method of making a more accurate wavefront measurement of a patient""s eye comprises illuminating the patient""s retina with an illumination beam over an optical path between the source and the patient""s eye that is free of any refracting, diffracting or phase altering components. An aspect of this embodiment involves providing foveal illumination with a beam diameter that is less than a diffraction limit value of an imaging component that images a portion of the wavefront on a detector over a refractive focus range of the patient""s eye between about xe2x88x9212D to +6D. The method further entails providing a Gaussian illumination beam having a Rayleigh range that is greater than the refractive focus range of the patient""s eye between about xe2x88x9212D to +6D.
These and other objects of the present invention will become more readily apparent from the detailed description to follow. However, it should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art based upon the description and drawings herein and the appended claims.